There is an acoustic technique among conventional methods for estimating the direction-of-arrival (DOA) of a sound (or sounds). In this technique, an array of microphones is set up at equally-spaced intervals, and the direction of a sound source, or the direction-of-arrival of a sound wave, is estimated from the phase differences of microphones with respect to a reference microphone. An example of such a technique is disclosed in the book entitled “Acoustic System and Digital Processing” by Juro Ohga, Yoshio Yamazaki, and Noboru Kaneda, Corona Publishing Co., 1995, the disclosure being hereby incorporated herein by reference.
There is, on the other hand, another method for estimating the direction of a sound source (hereinafter also referred to as “sound direction”) which does not use the phase differences of output signals of a plurality of microphones located at a measuring point. In this method, sound direction is estimated by arranging a plurality of microphone pairs on straight lines which bisect each other at right angles and finding a ratio between the arrival time difference (corresponding to the phase difference) between the two microphones of a pair and that of the other pair. Such a method is disclosed, for instance, in Japanese Unexamined Patent Application Publication No. 2002-181913, Japanese Unexamined Patent Application Publication No. 2006-324895, and Japanese Unexamined Patent Application Publication No. 2008-224259.
Specifically, as shown in FIG. 10, four microphones M1 to M4 are arranged in two microphone pairs, namely, a microphone pair (M1, M3) in range with each other at a predetermined interval and a microphone pair (M2, M4) in range with each other at the same predetermined interval, on two straight lines bisecting at right angles, and a horizontal angle θ formed between the measuring point and a sound source position is estimated from the ratio between the arrival time difference between the sound pressure signals entering the microphones M1 and M3 constituting the microphone pair (M1, M3) and that between the sound pressure signals entering the microphones M2 and M4 constituting the microphone pair (M2, M4). In addition, a fifth microphone M5 is placed in a position perpendicularly apart from the plane formed by the microphones M1 to M4 such that it further forms four microphone pairs (M5, M1), (M5, M2), (M5, M3), and (M5, M4), and an elevation angle φ between the measuring point and the sound source position is estimated from the arrival time differences between the microphones constituting the respective microphone pairs.
This arrangement enables estimation of sound direction with accuracy by fewer microphones than in the microphone array technique first mentioned herein.
Also, an image sampling means, such as a CCD camera, can be added to this arrangement. After an image in the estimated direction of a sound source is captured, the image data and the sound direction data may be combined with each other for a graphic representation of the estimated sound direction and the sound pressure level shown within the image. Such a graphic representation will help the user visually grasp the sound sources.
Also, in an actually practiced method, images are continuously picked up by the image sampling means simultaneously with the sampling of sounds, and the image information, together with the sound information, is stored in the computer as moving images. Then the estimation of a sound source is performed by calculating the sound direction and graphically displaying the sound direction and the sound pressure level in the images.
In the conventional method, however, after the sound information and the image information are loaded into the computer, all the information thus loaded is used in the calculation of the direction of the sound source. As a result, the analysis of the sound source position takes a considerable time, and it is not possible to see the result of the sound source estimation instantly on the spot. Also, another drawback of the method is its inability to verify the validity of the measurement taken.
The present invention has been made to overcome such drawbacks of the conventional method, and an object thereof is to provide a method and an apparatus for estimating a sound source position on the spot by extracting an optional portion of measured data.